Hello. Some of the best deals I've gotten on Hi Powers have been from buying used ones at a good price.
I'd suggest to look for the following:
1. What's the overall appearance of the pistol? A worn finish does not always indicate heavy use or neglect, but it might so it's something to check. Frequently, it might be like some LEO's guns, carried lots but shot little. Are the grips screws buggered up or are the slots straight and not marred indicating that either the previous owner cared enough to use the right size screwdriver or not removed the grips at all?
2. Is the magazine disconnect in place or not? This is very good to know for obvious reasons? If it is gone, is the frame buggered or dinged up near the trigger pin and is the small pin in the lower rear of the trigger in good shape? Press the pointed end of the trigger pin to be sure it's tightly in place.
3. Check the barrel-to-slide fit, i.e.; does the barrel show movement when in battery? Tight is almost always better than loose in this regard. Slide to frame play should be checked as well. There will almost always be some, but it should not be excessive.
4. Look at the breech face? If the finish is completely gone, the pistol's been fired quite a bit. Now that's not necessarily a bad thing, but if you've been told that the gun's only been fired a very few times, it gives an idea of whether or not you're being told the truth.
5. Is the muzzle crown clean or is it dinged/dented? If it is, you'll probably have to have it recrowned.
6. Has the feed ramp been throated? Again, not necessarily "bad," but usually not needed on MkIII pistols. If it appears highly polished, check the bottom of the chamber to make sure it has adequate support.
7. Making sure the pistol's NOT loaded, cock the hammer and check the trigger pull. Is it heavy or light, gritty or clean? This might give an idea of whether or not a trigger job's going to be needed.
8. Also, with the pistol cocked, engage the safety and then press the trigger. Don't exert so much pressure your knuckles turn white, but just a firm press. The hammer should not move. Now, disengage the safety and see if the hammer stays cocked and note if the sear has moved up and out of engagement even a little with the full-cock notch in the hammer.
9. IF you can let the slide slam shut on an empty chamber, not repeatedly, but once or twice; does the hammer stay cocked? It should. Also see if the sear is staying down and in place.
10. If possible, field strip the pistol and check the locking lugs. The edges should be square and sharp, not rounded off. Is the bore clean, but if dirty, is the rifling in good shape and the bore not pitted?
11. Check the extractor's spring tension. Push down on the back end of it. It should be pretty hard to move as HP extractors are sprung pretty hard. Is the ejector claw as it should be or is it chipped?
12. The ejector should be straight and the end square.
Other things to check include cracks in the slide and/or frame. On the slide, check the area behind the ejection port. On the frame, check the area around the slide stop lever hole and if you can remove the slide, check the cam. This is the bar of steel that the barrel sits on. Look at the firing pin stop. If it's cracked, it will probably be at the 7 O' Clock position.
Well, I've probably forgotten something, but hope this helps.